LASIK Fundamentals, Surgical Techniques, and Complications - part 2 doc

This heating can result in energy transduction into corneal surface shockwaves, an undesirable outcome.Currently there are three approaches to refractive photoablative decomposition: sca

85 A Maldonado-Bas, R Onnis Results of laser in situ keratomileusis in different degrees of opia Ophthalmology 1998;105(4):606–611.

my-86 S Goker, H Er, C Kahvecioglu Laser in situ keratomileusis to correct hyperopia from
4.25 to

8.00 diopters J Refract Surg 1998;14(1):26–30.

87 R Zaldivar, JM Davidorf, S Oscherow Laser in situ keratomileusis for myopia from 5.5 to

11.50 diopters with astigmatism J Refract Surg 1998;14(1):19–25.

88 DJ Salchow, ME Zirm, C Stieldorf, A Parisi Laser in situ keratomileusis for myopia and opic astigmatism J Cataract Refract Surg 1998;24(2):175–182.

my-89 RL Lindstrom, DR Hardten, YR Chu Laser in situ keratomileusis (LASIK) for the treatment of low, moderate, and high myopia Trans Am Ophthalmol Soc 1997;95:285–296; discussion 296–306.

90 GO Waring III, JD Carr, RD Stulting, KP Thompson Prospective, randomized comparison of simultaneous and sequential bilateral LASIK for the correction of myopia Trans Am Ophthal- mol Soc 1997;95:271–284.

91 SG Farah, DT Azar, C Gurdal, J Wong Laser in situ keratomileusis: literature review of a veloping technique J Cataract Refract Surg 1998;24(7):989–1006.

de-92 LJ Maguire Topographical principles in keratorefractive surgery Int Ophthalmol Clin 1991; 31:1–6.

93 JT Holladay, TC Prager, RS Ruiz, JW Lewis, H Rosenthal Improving the predictability of traocular lens power calculations Arch Ophthalmol 1986;104:539–541.

in-94 SM MacRae Supernormal vision, hypervision, and customized corneal ablation J Cataract fract Surg 2000;26(2):154–157.

Re-95 G Alessio, F Boscia, MG La Tegola, C Sborgia Topography-driven photorefractive tomy: results of corneal interactive programmed topographic ablation software Ophthalmology 2000;107(8):1578–1587.

keratec-96 M Mrochen, M Kaemmerer, T Seiler Wavefront-guided laser in situ keratomileusis: early sults in three eyes J Refract Surg 2000;16(2):116–121.

re-97 M Ito, AJ Quantock, S Malhan, DJ Schanzlin, RR Krueger Picosecond laser in situ atomileusis with a 1053-nm Nd:YLF laser J Refract Surg 1996;12:721–728.

Lasers in LASIK

Basic Aspects

RODRIGO TORRES

Massachusetts Eye and Ear Infirmary and Harvard Medical School,

Boston, Massachusetts, U.S.A.

ROBERT T ANG

Massachusetts Eye and Ear Infirmary and Harvard Medical School, Boston, Massachusetts, U.S.A., and Asian Eye Institute, Makati, The Philippines

DIMITRI T AZAR

Massachusetts Eye and Ear Infirmary, Schepens Eye Research Institute,

and Harvard Medical School, Boston, Massachusetts, U.S.A.

1 Definitions

Irradiance, mJ/cm2 Energy/surface area A measurement of the amount of light ergy striking a given surface area Conceptually it is the energy density transferredonto a surface from a laser pulse Irradiance is a useful term, frequently correlatedwith corneal ablation rate in refractive surgery

en-Fluence, mJ/cm3 Energy/volume Often confused with irradiance In refractivesurgery, fluence is not usually used to describe a laser When fluence is mentioned,irradiance is often what is truly implied

Energy, joule (J) Kgm2/s2 Also measured in electron volts (eV) A force through adistance, or work Energy is what is required for breaking chemical bonds

Power, watt (W) J/s Kgm2/s3 Energy/time The rate at which energy is delivered

Intensity, Watt/surface Area W/cm2 Power density along a surface

Homogeneity, the distribution of energy within the cross section of a laser beam If a

beam is perfectly homogeneous, the distribution of energy within the beam is form.

uni-Coherence, monochromatic light waves perfectly in phase and parallel.

femtosecond, 1015seconds; picosecond, 1012seconds; nanosecond, 10-9seconds

2 Brief History of Lasers

Laser technology is just over forty years old The theory of visible frequency lasers was firstproposed by Schawlow and Townes in 1958 when they expanded upon an existing theory

of microwave masers (1) In 1960, Maiman demonstrated the first experimental laser, aruby crystal laser powered by a flashlamp (2) Since then, laser technology has grown ex-ponentially

The word laser comes from the acronym LASER, standing for light amplification bystimulated emission of radiation The theory of stimulated emission is central to the gener-ation of the laser beam With stimulated emission, when a photon interacts with an excitedatom capable of emitting an identical photon, the atom is stimulated to emit the photon sothat it goes in the same direction and perfectly in phase with the incident photon In thisfashion, order is introduced to the normally chaotic process of electromagnetic emission Ifenough of these coherent photons are recruited from the excited atoms, a beam ofmonochromatic, unidirectional, and in-phase photons is produced

3 Laser Properties Relevant to Photorefractive Surgery

absorb the radiant energy (Fig 3.2) Other atoms are simply unaffected by the photons andlet them pass by By manipulating the frequency of light, we can choose whether we wantthe energy to affect water or biological tissue Effectively, we can pick our moleculartargets.

falling in the UV-C range of the spectrum The dominant chromophore in cornea for the

193 nm and 213 nm wavelengths within this range is the peptide bond linking adjacentamino acids in collagen (3) Photons at these wavelengths carry in the range of 6.4 eV ofenergy each, an amount exceeding that in typical peptide bonds The bonds are broken bythe photon interaction Ideally this process occurs with very little thermal generation, be-cause the energy is used primarily in breaking bonds The resulting fragments occupy agreater volume than the single polymer from which they originated and are imparted somekinetic energy from the irradiation Both of these factors contribute to supersonic ejection

of the material from the corneal surface (4,5) This process has been referred to as chemical ablation, ablative photodecomposition (6), and photoablation (7–9) The choice

of frequency determines whether photovaporization, photothermal shrinkage, or chemical ablation occurs

photo-b Homogeneity

A uniform distribution of energy in the beam cross section is termed beam homogeneity.Because no laser leaves the resonator with a uniform distribution, it is important to charac-terize the beam homogeneity In the absence of homogeneity, ablation rates are expected tovary over the treatment zone, resulting in irregular tissue removal (10) Prior to beam ma-nipulation, a gaussian distribution for the excimer laser is expected With gaussian beamprofiles, central overcorrection and peripheral undercorrection within the ablation zonehave been reported (11–13) These distortions have been associated with the use of smallablation diameters and a nonuniform gaussian beam profile which is hotter centrally(12,14)

Figure 3.2 The argon fluoride excimer laser demonstrates high absorption and low penetration properties within the corneal surface (From Ref 63.)

c Irradiance

Irradiance is central to studies of ablative threshold and ablative rate The photoablative

than this, appreciable ablation is not observed

Photoablative rate is also a function of the irradiance The relationship between lation rate and irradiance differs for different electromagnetic wavelengths At 193 nm, the

toablative rate tends to plateau for this wavelength (15) At 249 nm, however, the toablative rate does not plateau but continues to increase logarithmically as the irradianceincreases (Fig 3.3)

pho-The relationship between ablation rate and irradiance is important in refractivesurgery Because there is significant pulse-to-pulse variation in irradiance, the ablation ratemust be fairly consistent despite this variability The ablation rate to irradiance curve forthe 193 nm wavelength is favorable because this curve plateaus at an irradiance of 150

Another important property related to ablation rate is ablation efficiency, which is theablation rate divided by the irradiance Maximizing ablation efficiency minimizes excessenergy, which would otherwise contribute to shock waves, photochemical effects, and

d Intensity

Irradiance is a measure of the amount of energy imparted onto a surface area, and intensity

is a measure of the rate at which energy is imparted onto a surface area Imparting energy

at a rate greater than the rate at which it is absorbed by photoablation results in heating of

Figure 3.3 At irradiances greater than 150 mJ/cm 2 , the curve plateaus for the 193 nm wavelength The ablation rate remains fairly consistent despite a variability in irradiance (From Ref 15.)

collateral tissue This heating can result in energy transduction into corneal surface shockwaves, an undesirable outcome.

Currently there are three approaches to refractive photoablative decomposition: scanningslit, wide area (broad beam), and flying spot (Fig 3.4) A large diameter allows for the sim-pler wide area ablation approach, while smaller diameters rely on the scanning slit and fly-ing spot approaches

1 Wide Area (Broad Beam) Ablation

The wide area ablation method requires the use of a large-diameter beam and allows for multaneous treatment of the entire operating field in a pulsatile fashion By using di-aphragms, a certain area of the operating field can be shielded from laser ablation, allow-ing for control of myopic and astigmatic correction patterns of ablation when using thewide area approach This method has been in use longer than the other two and has gener-ated the most clinical data

si-Advantages of wide area ablation include a short operating time (often less than 30seconds for low myopic photorefractive keratectomy), obviating the need for eye tracking.Also, because the entire treatment area is ablated simultaneously, there is no need for so-phisticated scanning technology Only a computer-controlled variable iris diaphragm isneeded, which is simple to operate for the treatment of myopia and astigmatism

Disadvantages of wide area ablation center on the need to produce a stable, neous gaussian beam of large (about 6 mm) diameter in order to cover as much of the optic zone (treatable area) as possible This requires the use of an excimer laser head be-cause, to date, solid-state lasers cannot achieve ablative thresholds with homogeneous gaus-sian beams at such a large diameter (18) Excimer laser heads are large and bulky and useargon fluoride (ArF) gas, a toxic gas combination that introduces the risk of exposure Fur-thermore, maintaining beam uniformity and homogeneity at such large beam diameters ne-cessitates higher energy outputs, which translates to higher cost of operation Greater acous-tic shock waves are associated with the higher energy output Higher energy output also putsincreased strain on laser optics, increasing optical maintenance needs Correction of hyper-opia with the wide area approach has been technically challenging Asymmetric astigma-tisms cannot be corrected Wide field ablation has also been known to carry some risk ofsteep central islands, in contrast to scanning slit and flying spot approaches

homoge-Models available utilizing the wide field ablation approach include the VISX Star S2,Summit Apex Plus, Apex/OmniMed, and ExciMed; the Chiron-Technolas Keracor 116;and the Coherent-Schwind Keratom

2 Scanning Slit Ablation

Scanning slit ablation uses a rectangular beam of light that is passed unidirectionally overthe face of the cornea as laser pulses are passed through a slit-shaped diaphragm In thisfashion, a uniform layer of tissue can be removed from the cornea over the course of sev-eral pulses and slit positions This is in contrast to the wide area ablation, which can treatthe entire optical zone with each pulse

Advantages of the scanning slit approach result from the fact that the beam energydoes not have to be distributed over the entire treated area with each pulse Specifically, the

Figure 3.4 The three approaches to excimer laser photoablation (a) Scanning slit (b) Large or broad beam (c) Flying spot (From Ref 64.)

smaller beam allows for ablation to occur at energy outputs less than those required forwide field ablation Reduced acoustic shock waves and smoother ablative surfaces are pos-sible compared with wide area ablation Beam uniformity and homogeneity are much im-proved because of the smaller beam cross section The incidence of steep central islands ismuch lower using the scanning slit approach In contrast to wide area ablation, there are nooptical zone (the size of the treated area) limitations for photorefractive keratectomy (PRK)

or phototherapeutic keratectomy (PTK) with the scanning slit approach

Disadvantages of the scanning slit technique include increased dependence uponcomplicated scanning systems and a longer operating time, compared with wide field ab-lation Consequently, eye tracking and fixation become a concern with the scanning slit.Some systems using this approach include the Nidek EC-5000 and the Meditec MEL60

3 Flying Spot Ablation

This approach utilizes a tightly focused beam to ablate very small areas of the cornea at a

time The small beam is redirected with x and y axis mirrors Unlike the scanning slit

ap-proach, which requires scanning across the cornea in one direction, the flying spot is veryversatile and can be maneuvered in multiple directions

The small focal area of the flying spot permits ablation at lower energy outputs.This allows for smaller laser cavity size and reduced need for maintenance Acousticshock waves are reduced by lower energy outputs Homogeneity requirements are lessstringent, and fewer optics are needed for flying spot ablation Spot placement is versa-tile, allowing for complicated sculpting of the cornea This allows custom-designed pat-terns, permitting treatment of asymmetric astigmatism Hyperopic correction is facilitated

by this approach owing to the ease of creating peripheral annular ablation patterns Aswith the scanning slit approach, there are no optical zone limitations for PRK or PTK.Another advantage of the flying spot approach is the flexibility in laser heads Because awide area beam is not necessary for ablation, it is possible to achieve ablation using solid-state laser heads This offers the option of avoiding the risk of toxic gas exposure asso-ciated with excimer gas lasers

Disadvantages associated with the flying spot approach include a longer operatingtime and consequently the need for a sophisticated eye-tracking device, in addition to theneed for laser delivery (scanning) capability Longer operating time also results in varia-tions of corneal hydration, which has deleterious effects on surgical results

Overall, flying spot technology is young but shows great promise for the future in that

it allows for precise sculpting of the cornea If current disadvantages can be minimized withtechnological advances, this may become the approach of choice

Some of the flying spot lasers include the Bausch & Lomb Chiron Technolas 217,LaserSight Compak-200 Mini-Excimer Laser, Alcon Autonomous T-PRK and LadarVi-sion 4000, and Novatec LightBlade (a solid-state laser)

Laser heads can come in solid, liquid and gas phases Liquid lasers have no immediate plication to photorefractive refractive surgery Gas lasers investigated for refractive surgery

surgery

1 Gas Lasers—Excimer Lasers

Excimer lasers, developed for their ability to produce ultraviolet laser frequencies, are asubclass of the gas laser heads In 1975, excited xenon (Xe) atoms and halogen gas mix-tures were found to emit ultraviolet radiation following the dissociation of an unstablexenon halide intermediary By 1976, this type of interaction was noted with XeF, XeCl,XeBr, krypton fluoride (KrF), and (ArF) (19–22) The word excimer is a contraction of “ex-cited dimer.” Strictly speaking, a dimer is formed by two atoms of the same element Be-cause it was first thought that the intermediary structure for the excited state of these laserheads involved the formation of dimers (two atoms of the same element bound together),the term excimer was applied to these laser types Now it is recognized that the activatedintermediary is a rare gas halide, not a dimer The name grew popular, however, and is uni-versally accepted

The ArF excimer laser has a wavelength of 193 nm, which imparts an energy of 6.4

eV per photon This photon energy is adequate to break covalent bonds through the process

of ablative decomposition Krueger and colleagues compared the tissue effects of the ArF,KrF, XeF, and XeCl lasers and found that the 193 nm ArF wavelength provided forsmoother results, more precise ablation, and decreased thermal damage to adjacent tissue(23) For these reasons the excimer ArF laser is the laser of choice for LASIK and PRK

a Safety of ArF Excimer Lasers

Mutagenicity of the 193 nm wavelength Some frequencies of ultraviolet light carry

muta-genic potential Cyclobutyl pyrimidine dimers in DNA have been produced by irradiationwith wavelengths under 280 nm, making the question of mutagenicity of excimer radiation

a valid one (24) Results of a number of experimental studies have demonstrated that 193

nm radiation does not cause cytotoxic damage to DNA and mutagenicity (25–29)

In human skin, 193 nm ArF excimer radiation has led to no unscheduled DNA thesis activity, in contrast to 248 nm KrF irradiation, which does induce such activity (30).Unscheduled DNA synthesis activity can be suggestive of activated excision repair mech-anisms, commonly considered the most important mechanisms for removing damagedDNA In comparison with the 248 and 254 nm wavelengths, the 193 nm wavelength results

syn-in little damage to DNA, and damage syn-in 193 nm irradiated cornea is comparable to that syn-inunirradiated cornea (26) It has been suggested that the potential for cataractogenicity isalso very low, following the calculation of a very small lens exposure to excimer-inducedultraviolet fluorescence (31)

An explanation for the sparing of DNA damage by the 193 nm wavelength as posed to other far-UV wavelengths lies in the high absorption of this wavelength by cyto-

absorbed, effectively shielding the nucleus from damage Speculation of secondary UV posure is not of great concern because this level of exposure is 10,000 times lower than theannual exposure to solar UV radiation (32)

ex-Collateral damage by 193 nm wavelength ablative decomposition Because the

193 nm wavelength is such an accurate tool for photoablation, collateral damage is ated through indirect effects of the laser–tissue interaction Shock waves, particulate ejec-tion, and surface heating are the primary concerns in efforts to minimize collateral damage.Shock waves result from high-pressure, high-temperature, expanding gas cloudsabove the surface of the cornea They are estimated to travel at about 1 km/s, with varia-

medi-Figure 3.5 As laser energy is absorbed (a) peptide bonds are broken (b), resulting in increase in volume and subsequent ejection of particles (c) (From Ref 7.)

tions in velocity dependent on the energy of ablation and the wavelength of the radiation.Pressures of up to 100 atmospheres can be generated, making corneal damage possiblefrom shock waves Thermal denaturation of surface collagen can result in pseudomem-brane formation, currently considered a local protective effect (33) Structural changes be-neath the corneal surface following ablation include stromal vacuoles and an increasednumber of keratocytes in later stages of wound healing (34)

Particulate ejection occurs as ablative decomposition renders polymers into multipleheated polymeric fragments that occupy a larger volume (Fig 3.5) This forces particlesinto the air in a supersonic expanding plume (35) (Fig 3.6) Particles in the plume include

results in a recoil surface wave that travels at several meters per second, although as yet thiswave has not been demonstrated to induce corneal injury (38)

Corneal surface heating occurs secondary to ablative decomposition Theoretically,ablative decomposition channels energy into breaking peptide bonds and not into heat, butthe average corneal temperature has been observed to increase by 20°C during ablation(39) In theory, this increase in temperature could induce keratocyte injury

Accuracy of the 193 nm excimer beam ablation The 193 nm wavelength has

subcel-lular levels of accuracy for photoablative decomposition Typical ablation rates are

a red blood cell (7 m) of ablation per pulse In a dramatic demonstration of the precision

of the excimer laser, Srinivasan in 1983 used the ArF excimer to create grooves in a humanhair strand (Fig 3.7) Light microscopy of ablated areas show clean edges with no de-tectable traces of cellular injury other than in pale-staining cytoplasm of cells bordering theablation

One feature of this subcellular ablation is the formation of a pseudomembrane, whichcan be about 100 nm thick and is present at the ablation site There is no cell membrane atthis site, but the pseudomembrane is thought to provide a barrier to the passage of water aswell as possibly provide a scaffolding for wound reepithelialization On electron mi-croscopy the pseudomembrane appears as a condensation of electron-dense material (40)

Figure 3.7 The clean, sharp edges of the grooves created by the excimer laser on a human hair demonstrate the accuracy and precision of the laser (From Ref 5.)

Figure 3.6 During excimer laser ablation of the cornea, the ejection of particles into the air is ible as an expanding plume (From Ref 35.)

vis-Not all corneas form pseudomembranes It is not known what causes pseudomembrane mation, but the thermal effect and the uncoupling of organic double bonds during pho-toablation have been proposed as contributing to the phenomenon (41).

for-The 193 nm wavelength has the useful property of a relatively constant ablative rate

pulse-to-pulse variability in irradiance does not drastically affect the ablative rate, ing for predictable ablative depths

allow-b Other Advantages of the Excimer: Strong Beams with Large

Diameters

An important advantage of the excimer ArF laser is the ability to produce a homogeneous,large-diameter beam (~6 mm), at ablative irradiances, capable of treating the entire opticalzone simultaneously This was especially important earlier in photorefractive surgery wheneye tracking, eye fixation, and beam scanning made scanning slit and flying spot technolo-gies more challenging to develop As these technologies improve, reliance on large beamdiameters may diminish, opening the door to nonexcimer laser types that initially were im-practical for refractive purposes

c Disadvantages of Excimer Systems

Nonexcimer laser systems are desirable in the future owing to the several potential tages of excimer laser systems One disadvantage is the risk of toxic gas exposure The gasesare consumed by laser utilization and must be changed frequently, increasing the risk Thelarge size of the laser head and high energy requirements are also undesirable Optics forbeam homogenization can be complex and require frequent replacement Finally, because the

disadvan-193 nm wavelength is absorbed so avidly by cornea, the laser does not allow for intrastromalrefractive surgery This necessitates mechanical flap lifting for LASIK Although there is cur-rently no widely used alternatives to this approach for keratomileusis, in the future it would

be desirable to minimize the invasiveness of the procedure with intrastromal keratomileusis

2 Solid-State Lasers

Although the excimer gas lasers have made by far the most important contribution to photorefractive surgery, some solid-state lasers are showing promise for use in this capac-ity Keeping in mind that the goal for refractive surgery is to change the shape of the corneawith precision and little collateral damage, it is important to remember that ablative decomposition is only one way to attain this goal Any tissue-altering modality (photodis-ruption, photovaporization, collagen shrinking, photospallation) that alters tissue struc-ture in a precise, localized, and predictable fashion has potential as a tool for refractivesurgery

a Wavelength-Modulated Nd:YAG and Nd:YLF Lasers

The Nd:YAG and Nd:YLF lasers are infrared solid-state lasers The active media are theneodymium-doped crystals of either yttrium-aluminum-garnet or yttrium-lithium-fluoride.The Nd:YAG and Nd:YLF lasers have fundamental wavelengths of 1064 nm and 1053 nm,respectively When raised to the fifth harmonic, the wavelengths become 213 nm and 211

nm, respectively These are very close to the 193 nm ArF excimer wavelength, suggestingthat these lasers may be suitable for photoablative decomposition Histological studies

is similar to the results with the ArF laser (42) Furthermore, PRK with the 213 nm laserproduces smooth surfaces and transitions, with a reepithelialization rate and histopatho-logical findings similar to the results with the 193 nm excimer laser PRK in rabbit studies(43) This evidence supports the possibility that 213 nm photoablative decomposition isfeasible and safe.

b Mutagenicity of the 213 nm Wavelength

The question of mutagenicity at the 213 nm wavelength has been addressed by in vitro ies (44) These studies measured free radical production in bovine corneas and in vitro bac-terial survival following 193 and 213 nm irradiation at subablative irradiances Evidencesuggests that free radical production and species were identical for the two lasers, but the

stud-213 nm wavelength clearly demonstrated increased bacterial cytotoxicity If the differences

in cytotoxicity apply to mammalian cells, corneal fibroblasts responsible for collagen pair would be much more affected by 213 nm irradiation (45) In vivo studies in rabbits donot provide clear evidence as to the safety of the 213 nm wavelength in PRK In one study,the 213 nm radiation was found comparable to the 193 nm radiation (43), but in anotherstudy, the results were significantly worse at 213 nm (46) Increased postoperative compli-cations observed in the latter study could be partially explained if the 213 nm wavelengthwere cytotoxic to mammalian keratocytes More studies on the mutagenicity of the 213 nmwavelength will be an interesting area of future work

re-In summary, the frequency-quintupled solid-state lasers do show promise as tools forrefractive surgery A solid-state laser on the market functioning at the 211 to 213 nm wave-length range is the Novatec Lightblade flying spot laser Although these lasers are not

as energy efficient as excimers, they show comparable ablative efficiency As energyefficiencies of these systems improve, they may become available as wide-area ablationlasers, which would decrease operative time, improve intraoperative corneal hydration, andpossibly produce superior surgical results Also, as flying spot and eye tracking technologydevelop further, low-energy, small-beam solid-state lasers can also be expected to producesuperior results Although not as clinically developed, solid-state lasers have several ad-vantages over the excimers The frequency-quintupled Nd:YAG laser is less expensive andmore compact than the ArF laser and does not require the use of toxic gases Furthermore,solid-state lasers can provide increased reliability, robustness of design, safety, and loweroperating costs than gas or dye lasers

c Nd:Glass (Femtosecond) Lasers

The Nd:glass lasers have been investigated for their ability to generate femtosecond pulses.The technology for such generation has only recently become feasible Because thesepulses provide a much higher power output and a much shorter exposure time, they requireonly one-fourth to one-tenth the energy to produce corneal photodisruption compared withpicosecond and nanosecond infrared laser pulses (47,48) By utilizing less radiant energy,the secondary shock wave and cavitation bubble size are reduced, allowing for closer pack-ing of laser pulses and more contiguous photodisruption

In a study using femtosecond pulses from a Nd:glass laser to produce an intrastromal

below the surface epithelium of the cornea and scanned in a spiral pattern to create a plane(49) The plane was extended to the surface to create a flap, and mechanical tissue planeseparation was used to grade the contiguity of the intrastromal ablation The results indi-cated that many of the shortcomings of picosecond technology (including incomplete tis-

sue cutting, poor dissection, and surface quality) are significantly improved when the pulseduration is further reduced into the femtosecond region The femtosecond pulses allow for

place-ment of pulses and smoother tissue dissection (47) Although results are encouraging, invivo systems that closely reproduce the clinical situation must be investigated to establishclinical efficacy An in vivo study in rabbits concluded that intrastromal photodisruptionwith femtosecond lasers produced consistent changes in corneal thickness without loss ofcorneal transparency (50) The study did not evaluate changes in corneal topography orspecular microscopy, although no damage to the corneal endothelium was seen on histo-logic sections This laser is now being used to create LASIK flaps in patients undergoingLASIK surgery with good results

Advances in ultrafast laser design and the development of powerful laser diodes havefinally made low-cost, reliable, diode-pumped, femtosecond laser systems possible (51).Femtosecond lasers are solid-state, enjoying all of the advantages of solid-state lasers overexcimer lasers Because infrared radiation is transmitted by corneal tissue (unlike excimer

193 nm radiation), infrared femtosecond lasers could allow for intrastromal refractivesurgery that obviates the need for mechanical flap creation or epithelial disruption (50) Itwill be exciting to follow the advances in femtosecond laser technology and the applica-tions it offers for corneal flap cutting, laser keratomileusis, corneal implant placement, andpossible intrastromal keratectomy

SYSTEMS

1 Causes of Beam Degradation

Excimer laser systems are high-maintenance systems that require frequent calibration

to ensure high performance Unlike other ophthalmic lasers, there is no modification of the laser energy based on any clinical indication of tissue response to ablation This feature makes preoperative calibration an essential step in photorefractive surgery with theexcimer (52)

a Optics Degradation

Optics degradation is a natural consequence of laser energy interaction with the lens andmirror coatings This leads to deterioration of beam quality and irradiance Beam qualitycan be decreased by a reduction in beam homogeneity A beam that loses its homogeneity

is no longer an ideal surgical tool Hot spots within the beam reduce ablation surfacesmoothness and can lead to poor refractive outcomes Because broad beam systems havelarge beam diameters and high energy requirements to maintain beam homogeneity, theiroptical components can degrade fairly quickly Furthermore, broad beam systems rely oncomplex optics (compared with scanning systems) On the other hand, scanning systemsmust use far more pulses to ablate a corneal surface than the broad beam lasers, which taxesthe optics of the scanning systems For these reasons, it is important to monitor the condi-tion of optics with both broad beam and scanning systems

b Gas Impurities

Laser action releases contaminants into the laser head cavity The fluorine gas reacts withcontaminants within the laser cavity, further contaminating the cavity and reducing the

availability of fluorine for stimulated emission This reduces laser action To maintain rity, the gas must be either constantly cleaned or frequently replaced Cleansing of the gasinvolves precipitation of contaminants using a cryogenic device and liquid nitrogen Alter-natively, the chamber can be cleansed by flushing it with fresh gas to replace the contami-nated gas Improvements in laser construction have reduced the need for cryogenic gas pu-rification and prolonged the lifetime of a single gas fill (6).

pu-2 Calibration Techniques

It is important to check the laser ablation rate prior to surgery because if the output is toohigh, overcorrection is likely If the output is compromised, undercorrection is a risk Thischecking process is referred to as calibration Calibration methods are generally based onmeasurement of energy (energy models) or measurement of ablation (ablation models) En-ergy models use direct measurement of the energy output of the laser with meters, which is

Further-more, calibration of these meters requires sophisticated equipment that is difficult to useroutinely in the field For these reasons, the ablation models (ablation of Wratten filters,PMMA blanks, scanning profilometry) are used most frequently

One approach to calibration involves ablation of a piece of plastic, forming a lens.The plastic used is PMMA, or polymethyl methacrylate, the plastic used for contact lenses.The power of the formed lens can be tested with a lensometer to assess laser irradiance,laser alignment, and laser beam profile Distortions of the lens can indicate poor laser lightdistribution within the beam (6) Another approach to calibration involves counting thenumber of pulses necessary to perforate a plastic Wratten filter This approach can be mod-ified to ablate only 90% of the thickness of the plastic The quality of light transmissionthrough the plastic can be grossly assessed for uniformity

The VISX laser is calibrated based on preoperative ablations performed on a PMMAblock Then the laser is reprogrammed to adjust the ablation rate to the desired value TheSummit laser can be checked by doing a 90% gelatin film ablation The Chiron-Technolaslaser is reprogrammed after counting the number of pulses needed to perforate a specialfoil The Coherent-Schwind laser also uses a gelatin film for calibration, with reprogram-ming based on the number of pulses required for 10–90% perforation of the film TheLaserSight laser uses a patented device, ex-calibur, which employs commercially availablecorneal topography systems (54)

Broad beam excimer laser ablations are usually characterized by a relatively uniformdistribution of surface power within the treated zone (55) Nonetheless, the hypothesis thatcorneal stromal ablation shape exactly matches the laser beam profile has been shown to beinvalid (56) Rather, in uniform beams the ablation rate varies with the ablation diameter.This fact must be taken into account when attempting to calibrate by an energy model ap-proach Failure to correct for this variance can lead to central undercorrections and periph-eral overcorrections, a phenomenon termed steep central islands The underlying causes forspatially variable ablation rates across a uniform beam are unclear but may include differ-ential hydration of the cornea, redeposition of ablated material as it lifts from the surface,photospallation effects, dependence upon proximity to an unablated area, and persistent ab-lation fog overlying the stromal surface

Modifying the ablation algorithm to accommodate the spatial variance of corneal lation has produced spherical enucleated eye PRK ablations By analyzing the postopera-tive topography of PRK patients, the ablation algorithm was modified to produce a morespherical ablation in situ (56)

ab-E LASERS IN EYE TRACKING SYSTEMS FOR LASIK, LASEK, AND

PRK

Eye tracking is of fundamental importance to the various ablation approaches because it lows for superior control of the surgical field By tracking eye motion, the laser path can beredirected to ablate the desired portions of the cornea, regardless of involuntary saccades.This is especially important for the flying spot approach, where the beam is focused to ab-late only a very small portion of the cornea at a time and where ablation of the entire corneacan take several minutes Eye tracking is also especially important for complex asymmet-rical ablation patterns, where each cornea can be custom-corrected based on regional ir-regularities as opposed to gross topographic irregularities Furthermore, solid-state lasers,which show some promise of eventually replacing the bulkier, more dangerous excimerlasers, will most likely depend on flying spot approaches to ablation because of their lim-ited beam diameter In the future they will most likely rely heavily on adequate eye track-ing technology to produce clinically maximized results For these reasons, excellent eyetracking technology is a gateway to significant advances in photorefractive surgery.Eye tracking can be active, passive, or a mixture of the two Active eye tracking in-volves detection of a change in eye position It also involves compensating for the change

al-by redirecting the ablation laser beam to ablate the desired area, in its new location Passiveeye tracking is much simpler Here a change in eye position is detected, and if the change

is greater than a maximum acceptable limit, the laser ablation ceases until the eye is tered Then, ablation resumes where it had ceased The speed at which changes in eye po-sition can be detected and corrected is dependent upon the frequency at which the eye po-sition is sampled and monitored Frequency of sampling is determined by the type oftechnology used to monitor eye position

recen-Lasers can play an important role in the eye tracking systems used for LASIK,LASEK, and PRK The laser head used for eye tracking is not any of the types we have con-sidered previously for corneal ablation, quite simply because no ablation action isnecessary All the laser must do is detect changes in the eye position; no special laser-tissue interactions are necessary By using a laser to monitor eye position, the frequency ofsampling is very high, and eye tracking is much faster

Traditionally, eye tracking has been performed by infrared video cameras The quency of sampling with this approach is limited by the camera frame capacity (images ofthe pupil’s location are produced 60–120 times per second) Because lasers are capable ofsuch high-frequency pulsing, the sampling frequency of a laser-based approach is muchgreater One system, the Autonomous LADARVision excimer laser, uses this approach foreye tracking The term LADAR stands for laser radar This system uses a 905 nm diodelaser to measure the position of the dilated pupillary edge at a sampling rate of 4,000 times

fre-per second (57) This signal is then conveyed to x and y axis tracking mirrors Together the

laser and mirrors can react with a response time of 600 radians per second This means thatredirection of the ablation beam to hit the desired target on the cornea can occur within 10

to 25 milliseconds of the change in eye position This allows even the highest speed cadic eye movements (up to 700°/s or approximately 150 mm/s) to be tracked with a peak

de-tect and initiate a response over 50 times during a saccade and redirect the beam to its propriate location before a new change occurs This eye tracking feature is distinct fromothers in that it is mandatory for laser ablation function with the Autonomous LADARVi-sion excimer laser A disabled tracker will cause ablation to cease In other tracking sys-tems, the tracking function is optional Similar tracking systems are available on most cur-

ap-rent laser systems, but the tracking is based on lower sampling frequencies, and withequally good outcomes.

A recent study used the LADARVision system to treat one eye of each of 102 patients

follow-up results reported uncorrected visual acuity of 20/40 or better in 99% of eyes,20/20 or better in 70% of eyes, and BCVA of 20/25 in 100% of eyes One eye lost two lines

of BCVA (20/12.5 preoperative to 20/20 postoperative) No eyes lost more than two lines

this study were at least comparable, if not better, than those of a previous wide beam

(57,61) The tracking systems on other laser platforms have also shown improved tion and improved visual outcomes (See ch 14)

centra-REFERENCES

1 AL Schawlow, CH Townes Infrared and optical masers Phys Rev 1958;112:1940–1949.

2 TH Maiman Stimulated optical radiation in ruby Nature 1960;187:493–494.

3 S Lerman Radiant Energy and the Eye New York: Macmillan, 1980, pp 43–59.

4 BJ Garrison, R Srinivasan Microscopic model for the ablative photodecompensation of mers by far ultraviolet radiation (193 nm) Appl Phys Lett 1984;44:849.

poly-5 HH Jellinek, R Srinivasan Theory of etching of polymers by far-ultraviolet, high-intensity pulsed laser and long-term irradiation J Phys Chem 1984;88:3048.

6 S Trokel History and mechanism of action of excimer laser corneal surgery In: JJ Salz, ed Corneal Laser Surgery St Louis: Mosby, 1995, pp 1–10.

7 R Srinivasan Kinetics of the ablative photodecomposition of organic polymers in the far violet (193 nm) J Vac Sci Technol B 1983;1:923–926.

ultra-8 T Keyes, RH Clarke, JM Isner Theory of photoablation and its implications for laser totherapy J Phys Chem 1985;89:4194–4196.

pho-9 JM Krauss, CA Puliafito, RF Steinert Laser interactions with the cornea Surv Ophthalmol 1986;31:37–53.

10 PS Hersh, MD Wagoner Excimer Laser Surgery for Corneal Disorders New York: Thieme,

13 T Seiler, A Holschbach, M Derse, B Jean, U Genth Complications of myopic photorefractive keratectomy with the excimer laser Ophthalmology 1994;101:153–160.

14 Summit Technology, ExciMed UV200LA Laser System User’s Manual Waltham, MA: mit Technology, 1991, pp 5–25.

Sum-15 RR Krueger, SL Trokel Quantitation of corneal ablation by ultraviolet laser light Arch thalmol 1985;103:1741–1742.

Oph-16 EA Davis, JC Abad, DT Azar Excimer laser physics and biology In: DM Albert, FA Jakobiec, eds Principles and Practice of Ophthalmology 2 nd ed Philadelphia: WB Saunders, 2000, pp 588–600.

17 HJ Huebscher, U Genth, T Seiler Determination of excimer laser ablation rate of the human cornea using in vivo Scheimpflug videography Invest Ophthalmol Vis Sci 1996;37:42–46.

18 GT Dair, WS Pelouch, PP van Saarloos, DJ Lloyd, SM Paz Linares, F Reinholz Investigation

of corneal ablation efficiency using ultraviolet 213-nm solid state laser pulses Invest mol Vis Sci 1999;40:2752–2756.

Ophthal-19 SK Searles, GA Hart Stimulated emission at 281.8 nm from XEBR Appl Phys Lett 27:243–245, 1975

20 JJ Ewing, CA Brau Laser action on Sigma 2
12] Sigma 2
12 Appl Phys Lett 1975;27:350–352.

21 CA Brau, JJ Ewing 354-nm laser action on XEF Appl Phys Lett 1975;27:435–437.

22 JM Hoffman, AK Hays, GC Tisone High-power UV noble-gas–halide lasers Appl Phys Lett 1976;28:538–539.

23 RR Krueger, SL Trokel, HD Schubert Interaction of ultraviolet laser light with the cornea vest Ophthalmol Vis Sci 1985;26:1455–1464.

In-24 BM Sutherland, LC Harber, IE Kochevar Pyrimidine dimer formation and repair in human skin Cancer Res 1980;40:3181–3185.

25 S Jain, TW Hahn, RL McCally, DT Azar Antioxidants reduce corneal light scattering after cimer keratectomy in rabbits Laser Surg Med 1995;17(2):160–165.

ex-26 RC Nuss, CA Puliafito, E Dehm Unscheduled DNA synthesis following excimer laser ablation

of the cornea in vivo Invest Ophthalmol Vis Sci 1987;28:287–294.

27 J Trentacoste, K Thompson, RK Parrish II, A Hajek, MR Berman, P Ganjel Mutagenic potential of a 193-nm excimer laser on fibroblasts in tissue culture Ophthalmology 1987;94: 125–129.

28 CE Van Mellaert, L Missotten On the safety of 193-nanometer excimer laser refractive corneal surgery Refract Corneal Surg 1992;8:235–239.

29 IE Kochevar Cytotoxicity and mutagenicity of excimer laser radiation Lasers Surg Med 1989;9:440–445.

30 HA Green, R Margolis, J Boll, IE Kochevar, JA Parrish, AR Oseroff Unscheduled DNA thesis in human skin after in vitro ultraviolet-excimer laser ablation J Invest Dermatol 1987;89:201–204.

syn-31 MN Ediger Excimer-laser-induced fluorescence of rabbit cornea: radiometric measurement through the cornea Lasers Surg Med 1991;11:93–98.

32 HR Taylor, SK West, FS Rosenthal, B Muñoz, HS Newland, H Abbey, EA Emmett Effect of ultraviolet radiation on cataract formation N Engl J Med 1988;319:1429–1433.

33 J Marshall, S Trokel, S Rothery, H Schubert An ultrastructural study of corneal incisions duced by an excimer laser at 193 nm Ophthalmology 1985;92:749–758.

in-34 J Marshall, S Trokel, S Rothery, RR Krueger Photoablative reprofiling of the cornea using an excimer laser: photorefractive keratectomy Lasers Ophthalmol 1986;1:21.

35 CA Puliafito, D Stern, RR Krueger, ER Mandel High-speed photography of excimer laser lation of the cornea Arch Ophthalmol 1987;105:1255–1259.

ab-36 O Kermani, HJ Koort, E Roth, MV Dardenne Mass spectroscopic analysis of excimer laser lated material from human corneal tissue J Cataract Refract Surg 1988;14:638–641.

ab-37 G Kahle, H Städter, T Seiler, T Wollensak Gas chromatographic and mass spectroscopic ysis of excimer and erbium: yttrium aluminum garnet laser-ablated human cornea Invest Oph- thalmol Vis Sci 1992;33:2180–2184.

anal-38 Z Bor, B Hopp, B Rácz, G Szabó, I Ratkay, I Süveges, A Füst, J Mohay Plume emission, shock wave and surface wave formation during excimer laser ablation of the cornea Refract Corneal Surg 1993;9(suppl):S111–S115.

39 MW Berns, LH Liaw, A Oliva, JJ Andrews, RE Rasmussen, S Kimel An acute light and tron microscopic study of ultraviolet 193-nm excimer laser corneal incisions Ophthalmology 1988;95:1422–1433.

elec-40 J Marshall, S Trokel, S Rothery, RR Krueger A comparative study of corneal incisions induced

by diamond and steel knives and two ultraviolet radiations from an excimer laser Br J thalmol 1986;70:482–501.

Oph-41 S Trokel Evolution of excimer laser corneal surgery J Cataract Refract Surg 1989;15:373–383.

42 Q Ren, RP Gailitis, KP Thompson, JT Lin Ablation of the cornea and synthetic polymers ing a UV (213 nm) solid-state laser IEEE J Quant Electron 1990;26:2284–2288.

us-43 Q Ren, G Simon, J-M Legeais, J-M Parel, W Culbertson, J Shen, Y Takesua, M Savoldelli traviolet solid-state laser (213-nm) photorefractive keratectomy: in vivo study Ophthalmology 1994;101:883–889.

Ul-44 MN Ediger, GH Pettit, LS Matchette In vitro measurements of cytotoxic effects of 193 nm and

213 nm laser pulses at subablative fluences Lasers Surg Med 1997;21:88–93.

45 DM Maurice The cornea and sclera In: H Davson, ed The Eye Vol 1 New York: Academic Press, 1962, pp 289–368.

46 PG Soderberg, T Matsui, F Manns, J-H Shen, J-M Parel, J-M Legeais, M Savoldelli, I Drubaix,

M Menashe, G Renard, Y Pouliquen Three month follow up on changes in the rabbit cornea after photoablation with a pulsed scanning beam at 213 nm In: JM Parel, Q Ren, KM Joos, eds Ophthalmic Technologies V Proc 1995;SPIE 2393:55–60.

47 T Juhasz, GA Kastis, C Suárez, Z Bor, WE Bron Time-resolved observations of shock waves and cavitation bubbles generated by femtosecond laser pulses in corneal tissue and water Lasers Surg Med 1996;19:23–31.

48 RM Kurtz, X Liu, VM Elner, JA Squier, D Du, GA Mourou Photodisruption in the human cornea as a function of laser pulse width J Refract Surg 1997;13:653–658.

49 RM Kurtz, C Horvath, H-H Liu, RR Krueger, T Juhasz Lamellar refractive surgery with scanned intrastromal picosecond and femtosecond laser pulses in animal eyes J Refract Surg 1998;14:541–548.

50 KR Sletten, KG Yen, S Sayegh, F Loesel, C Eckhoff, C Horvath, M Muenier, T Juhasz, RM Kurtz An in vivo model of femtosecond laser intrastromal refractive surgery Ophthalmic Surg Lasers 1999;30:742–749.

51 A Braun, H Liu, C Harvath, X Liu, T Juhasz, G Mourou All solid-state, directly diode-pumped chirped-pulse amplification laser system OSA Technical Digest 1997;11:323–324.

52 RR Krueger Excimer laser: a step up in complexity and responsibility for the ophthalmic laser surgeon? J Refract Corneal Surg 1994;10:83–86.

53 JD Gottsch, EV Rencs, JL Cambier, D Hall, DT Azar, WJ Stark Excimer laser calibration tem J Refract Surg 1996;12:401–411.

sys-54 P Asbell, CF Beyer, MP Nevitt LaserSight Compak-200 Mini-excimer laser In: JH Talamo,

RR Krueger, eds The Excimer Manual: A Clinician’s Guide to Excimer Laser Surgery Boston: Little, Brown, 1997, pp 341–353.

55 SD Klyce, MK Smolek Corneal topography of excimer laser photorefractive keratectomy J Cataract Refract Surg 1993;19:122–130.

56 JK Shimmick, WB Telfair, CR Munnerlyn, JD Bartlett, SL Trokel Corneal ablation etry and steep central islands J Refract Surg 1997;13:235–245.

profilom-57 RR Krueger In perspective: eye tracking and autonomous laser radar J Refract Surg 1999; 15:145–149.

58 M McDonald, LC Van Horn Autonomous T-PRK In: JH Talamo, RR Krueger, eds The cimer Manual: A Clinician’s Guide to Excimer Laser Surgery Boston: Little, Brown, 1997, pp 355–368.

Ex-59 D Boghen, BT Troost, RB Daroff, LF Dell’Osso, JE Burkett Velocity characteristics of normal human saccades Invest Ophthalmol Vis Sci 1974;13:619–623.

60 IG Pallikaris, KI Koufala, DS Siganos, TG Papadaki, VJ Katsanevaki, V Tourtsan, MB Donald Photorefractive keratectomy with a small spot laser and tracker J Refract Surg 1999;15:137–144.

Mc-61 VISX Excimer Laser System PRK Professional Use Information Manual, 1998.

62 DS Gartry The development of excimer laser corneal surgery: beam tissue interactions In: CNJ McGhee, HR Taylor, DS Gartry, SL Troke, eds Excimer Lasers in Ophthalmology: Principles and Practice Boston: Butterworth-Heinemann, 1997, p 33.

63 DS Gartry The development of excimer laser corneal surgery: beam tissue interactions In: CNJ McGhee, HR Taylor, DS Gartry, SL Trokel, eds Excimer Lasers in Ophthalmology: Principles and Practice Boston: Butterworth-Heinemann, 1997, p 33.

64 PP van Saarloos Physical principles of excimer laser In: CNJ McGhee, HR Taylor, DS Gartry,

SL Trokel, eds Excimer Lasers in Ophthalmology: Principles and Practice Boston: worth-Heinemann, 1997, p 18.

Massachusetts Eye and Ear Infirmary, Schepens Eye Research Institute,

and Harvard Medical School, Boston, Massachusetts, U.S.A.

Microkeratomes have evolved from suction rings and moving blades that were difficult tooperate to more user-friendly devices currently utilized in LASIK refractive surgery Theearly microkeratomes used by José Ignacio Barraquer were limited to the treatment of highmyopia Barraquer’s technique, involving the creation of a central lenticule with a manualmicrokeratome, freezing this section, and carving the deep portion with a lathe, according

to the amount of refractive error, had several technical limitations and was associated withnumerous complications Complications of this technique resulted from difficulties in mas-tering the microkeratome and cryolathe and included lenticular damage, persistent cornealhaze, irregular astigmatism, under- and over-correction, and regression (1) Innovations toaddress this problem, including Swinger’s technique, in which he did not freeze the backside of the lenticule (1–4), and Ruiz’s in situ method for manually removing corneal stromadeep to the flap (5), were particularly important milestones that led to the development ofcurrent microkeratomes

Burrato used argon-fluoride excimer laser ablation, previously reported for surfaceablation by Trokel (6), in combination with the microkeratome on the deep portion of thelenticule (7) Pallikaris demonstrated that in situ ablation and refractive correction could be

performed on the remaining stromal bed (8) With these advances and the increasing ularity of the excimer laser, the microkeratome still had its place, but it was no longer usedfor reshaping Instead, its primary purpose was the creation of the corneal flap.

pop-The microkeratomes of today have become safer and have been proven successful inperforming corneal lamellar cuts; however, complications with their use continue to occur.These complications have been decreasing with surgical experience, advancements, anddesign elements incorporated into newer microkeratomes The current microkeratomes are

by no means close to being free of complications, nor do they approximately an ideal crokeratome, which would optimize the following parameters: safety, ease of use and ster-ilization, reliability, moderate increase in IOP and minimal structural deformation duringsurgery, smooth beds, regular edges, reproducible thickness, sufficient hinge width, suctionring stability, and reasonable cost of hardware and disposables Although currently usedmicrokeratomes incorporate several features of the ideal microkeratome, they neverthelesssacrifice some obviously useful features in order to achieve others

mi-Typical components of a modern microkeratome system include the following (9):Motor

Microkeratome head

Applanator lenses to measure the diameter of the exposed cornea

Vacuum fixation ring used to secure the eye

Flap stop ring, which limits the travel of the microkeratome head through the fixationring

Automated horizontal microkeratomes include the Bausch and Lomb Surgical/Chiron tomated Corneal Shaper; Summit Krumeich-Barraquer (SKBM); Herbert Schwind; B.B.-I-T-I, Allergan, and Nidek (similar to Summit), Med-Logics, LaserSight Technologies Ul-trashaper, and Innovative Optics Innovatome Manual microkeratomes in this category

Au-include the Moria LSK-1, the SCMD Turbokeratome by New United Development poration, and the Med-Logics Manual.

Cor-The automated sliding microkeratomes (SKBM, B.B.-I-T-I, Allergan, and Nidek)have virtually displaced all other horizontal microkeratomes in this category because theyare surgeon-independent reliable units that require minimal assembly during surgery andhave relatively high reproducibility of flap thickness

1 Basics

Horizontal microkeratomes are derived from the original design by Barraquer During thekeratomileusis procedure, the turbines or electric motors of these instruments are locatedhorizontally or to the side of the eye Cuts are usually made with nasal hinges (Fig 4.1A),although it is possible to position narrow versions of these microkeratomes to create supe-rior hinges

Figure 4.1 Hinge location (A) Nasal hinge The keratomileusis occurred in the temporal-to-nasal direction (B) Superior hinge.

Figure 4.2 Gears vs sliding mechanism in nondisposable horizontal microkeratomes (A) The gears of the ACS head move through the geared tracks of the suction ring (B) The SCMD slides through its track without the assistance of gears.

Figure 4.3

operating surface (A) Moria Lamellar System for Keratoplasty-1 (B) Summit Krumeich-Barraquer (C) Med-Logics Microkeratome (D) Innovative Optics Innovatome (E) LaserSight Technologies Ultrashaper.

2 Design

Automated horizontal microkeratomes may advance through the use of rotating gears tween the microkeratome head and the suction ring (Fig 4.2A) or through gearless sliding ofthe microkeratome head on the suction ring Manual microkeratomes primarily consist ofgearless sliding of the microkeratome head on the suction ring (Fig 4.2B)

be-Gear-driven microkeratomes may have an increased likelihood of jamming due tooperative debris and may also be more difficult to clean between procedures Sliding de-signs may decrease these complications The instances of microkeratome head slippagefrom the suction ring tracks in the manual microkeratomes have been practically eliminated

in the newer automated sliding horizontal designs

The ACS is a gear-driven nondisposable horizontal microkeratome, while the other struments in this category utilize the sliding mechanism (Figs 4.2B and 4.3) The MoriaLSK-1 and the SCMD, both manual microkeratomes, run on a gas-turbine mechanism, whilethe others use an electric motor Multiple flap sizes and depths are available Technical fea-tures of individual nondisposable horizontal microkeratomes may be seen in Table 1

per-Table 1 Technical Features of Certain Nondisposable Horizontal Microkeratome

Model Name Automated Krumeich-Barraquer Med-Logics Ultrashaper

Corneal Shaper

Oscillation speed 15,000 rpm 12,000 rpm 9,000 rpm 9,000 to 20,000 rpm Flap diameter 7.5 to 8.5–10.0 9.0 or 10.0 8.5 or 9.55

Depth 130, 160, 180 m 130, 168, 180 130, 160, 180 140, 160, 180 Mechanism Sliding Track & Reel Dual Motors Proprietary Automated/manual Manual Automated Automated Automated

in the use of all microkeratomes to ensure that a variable depth plate is tightened to theproper level or that fixed depth plates are always inserted prior to keratomileusis.

4 Oscillation and Advancement Rate

Automated microkeratomes allow advancement at a constant, appropriately slow speedthroughout the keratomileusis procedure Slow advance rates allow the blades to performtheir shearing action with decreased chance of scraping, plowing, or producing areas ofthe flap that are thinner than others When using manual microkeratomes, the surgeonmust maintain a slow, steady advance rate throughout the cut to ensure consistent flapthickness

Fast oscillation speeds allow for a cut that has reduced ridges or “chatter” in the mal bed, while slow oscillation speeds have increased ridges Gas turbines have allowedmicrokeratomes to oscillate at faster rates; however, they require large gas canisters to re-main in the operating room

stro-Postoperative irregular astigmatism may be caused by variation in thickness out a flap or increased ridges created in the corneal stroma Therefore, to minimize inducedastigmatism, the surgeon should aim for a consistent flap thickness and minimal ridges or

through-“chatter” especially when using a manual unit (10,11)

The manual horizontal microkeratomes that we have studied, Moria and SCMD, tempt to decrease the chance of induced astigmatism with a faster oscillation speed, whilethe automated microkeratomes decrease the complication by a programmed advance rate

at-It may be appropriate to hypothesize that the chance of induced astigmatism can be mized by combining a fast oscillation with a consistent advance rate, which is the guidingprinciple of most of the newer automated sliding units

These microkeratomes are available in automated and manual forms The automated crokeratomes include the Carriazo-Barraquer and M-2 made by Moria and the Bausch andLomb Surgical/Chiron Hansatome Moria/Schwind also make manual versions of the Car-riazo-Barraquer microkeratome

mi-1 Basics

In nondisposable vertical microkeratomes, the motor lies directly vertical or above the eye,rather than to the side of the eye This instrument may be used to create a superior hinge(Fig 1B) Vertical microkeratomes are equipped with a rotating head that travels to createthe surgical arc (flap)

2 Designs

The primary difference between the two popular vertical microkeratomes is the presence

of a geared track on the Hansatome compared to the smooth track of the quer (Fig 4.4) As with the ACS, the geared track may cause this instrument to be moresusceptible to jamming from debris; however, it provides the microkeratome with more

keratomileusis with minimal assistance from the surgeon The Carrazo-Barraquer’s lack

Figure 4.4 Examples of nondisposable vertical microkeratomes Note that the microkeratomemotor is directly above the operating surface (A) The geared outer track of the Hansatome suction ring (B) The smooth outer track of the Carriazo-Barraquer suction ring (C) Fully assembled Hansatome with suction ring, plate, and head (D) Fully assembled Carriazo-Barraquer with suction ring and head Note the microkeratome head in B is smaller than A The M-2 Moria Microkeratome has virtually replaced the Carriazo-Barraquer microkeratome because it avoids the problem of flap thickness variability.

Table 2 Technical Features of Nondisposable Vertical Microkeratomes

Manufacturer Bausch & Lomb/Chiron Moria and Schwind